Method of making electroluminescent display panel with enlarged active display areas

ABSTRACT

An improved electroluminescent display panel having an X-Y array of display elements upon a planar insulating substrate. Integral thin film transistor circuit elements and drive signal buses are interconnected on the panel with individual electroluminescent electrodes covering a large area of the panel to increase the active display area. The electroluminescent electrode is a multilevel electrode with a first level portion disposed on the insulated substrate, a second level electrode portion disposed over an insulative polymerized layer which covers the thin film circuit areas and the drive signal buses, and a connecting electrode portion which extends between the first and second level electrode portions.

This invention was made under a contract with the Department of theArmy.

This is a division of application Ser. No. 636,281 filed Nov. 28, 1975now U.S. Pat. No. 4,006,383.

BACKGROUND OF THE INVENTION

The present invention relates to flat panel display devices and moreparticularly flat panel electroluminescent display devices. In suchdevices an X-Y array of display elements or cells, are provided upon aninsulated substrate, and are interconnected together to produce a largearea flat panel display which is substitutable for a cathode ray tube.Each of the display elements of the array comprises integral thin filmtransistor switching and control circuit elements, which are used toselectively address specific areas of the planar electroluminescentphosphor layer which is excited to produce light output in a displaypattern.

Such an electroluminescent display panel is described in U.S. Pat. No.4,042,854 entitled, "Flat Panel Display Device With Integral Thin FilmTransistor Control System." As described in the copending application,the electroluminescent display panel is fabricated by vacuum depositingsequential layers of selected materials to form the X-Y array of displayelements on an insulative substrate. Each display element covers anequal area of the panel, and a substantial portion of the area of thedisplay element is occupied by the individual thin film circuit elementsand particularly by the requisite spacing between such elements toprevent unwanted electrical interaction between the elements. For highresolution applications the physical size and area of this displayelement must be reduced, and this further increases the percent area ofeach display element taken up by the thin film circuit elements asopposed to the electroluminescent electrode. This electrode is the onlyportion of the display element which actually excites theelectroluminescent phosphor which is disposed uniformly over the panel.The actual size of the thin film circuit elements cannot readily bereduced because of the need to maintain desired electricalcharacteristics. This is particularly true with respect to the storagecapacitive element which is required in one embodiment of the addressingcircuit utilized for such an electroluminescent display panel. In orderto achieve a large enough capacitive value for this storage capacitor,its effective area is relatively large.

In the above described copending application, a technique foreffectively isolating the electroluminescent phosphor layer from thethin film circuit elements and the drive signal buses is set forth. Alaminated photopolymerizable layer is provided over such thin filmcircuit elements and the signal buses to thus effectively isolate theelectroluminescent phosphor from these electrical components. Thislaminated photo-polymerizable layer is applied in a relatively thicklayer with the photo-polymerizable insulative layer being selectivelyremoved from the areas over the electroluminescent electrode to permitcontact of such electrode with the phosphor layer which is thendeposited over such electrodes and over the insulative polymerizedportions which cover the thin film circuitry and signal bus elements.

The brightness and resolution of such electroluminescent display panelshas been limited by the effective area of the electroluminescentphosphor layer which is in contact with and excited by individualelectroluminescent electrodes. Till now this lit area has been aboutfifteen percent of the panel area. It is, therefore, highly desirablethat the electroluminescent electrodes be extended to cover a greaterarea of the total panel area.

SUMMARY OF THE INVENTION

An electroluminescent display panel structure is set forth in which theindividual electroluminescent electrodes are extended over a substantialarea of the total display panel. The individual electroluminescentelectrode extends from the insulative substrate and covers a substantialportion of the insulative polymerized layer above the thin film circuitportions of the display element. The individual electroluminescentelectrodes are comprised of a multi-level electrode with a first levelelectrode portion disposed on the insulative substrate, and with asecond level electrode portion disposed on the insulative polymerizedlayer, with a connecting electrode portion extending between the firstand second level electrode portions.

A preferred method of insuring deposition of a continuous connectingelectrode portion is set forth.

The effective lit area and brightness of the panel can thus be greatlyincreased, with a recent panel lit area being greater than about seventypercent of the panel area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of electroluminescent display panelof the present invention connected to the drive means;

FIG. 2 is a cross-sectional view through a portion of the panel whichillustrates the multi-level electrode structure of the panel of thepresent invention;

FIG. 3 is an enlarged schematic representation of the display elementarray pattern illustrating the thin film circuitry of the display panel;

FIG. 4 is a schematic illustration of a fabrication technique utilizedin fabricating the panel of the present invention;

FIG. 5a is an illustration of the edge structure of thephoto-polymerized layer when fabricating using prior art technique;

FIG. 5b is an illustration of the edge pattern of the photo-polymerizedlayer when fabricating using the method of the present invention asillustrated in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

the electroluminescent display panel 10 is seen schematically in FIG. 1connected to operative drive circuitry. The display panel 10 comprises aplanar insulating substrate 12 upon which are disposed thin film circuitelements 14 which are arrayed as X-Y rows and columns of individuallyaddressible and controllable display elements which are interconnectedby addressing and drive signal buses 16, 18 and 20. The basic thin filmtransistor circuit and method of fabrication is set forth in copendingapplication discussed above which is herein incorporated by reference.Each display element as seen more clearly in FIG. 3 includes a switchingtransistor T₁, a drive transistor or power transistor T₂, and a storagecapacitor C_(s). The video signal impressed on the X_(i) bus from analogvideo register 22 and line write scan means 24 to which the video signalis fed through switching transistor T₁ when the appropriate addressingsignal is present on switching bus Y_(j) turning on transistors T₁ tocharge the storage capacitor C_(s) to a voltage level which isindicative of the video signal. The switching buses Y_(j) areindividually connected to vertical scan driver 26. A high frequencypower supply 28 is connected to a common light transmissive, topelectrode above the electroluminescent phosphor to actuate the phosphor.When all of the capacitors C_(s) in a given line are charged with thevideo signal a gate signal is applied to the gate of transistor T₂turning T₂ on, and permitting application of the high frequency displaypower signal across the electroluminescent electrodes and the ELphosphor layer.

The thin film transistor circuit elements T₁, T₂, C_(s), and theinformation signal, switching signal and power signal buses aredeposited in successive stages as thin films of the respective materialswith appropriate insulation layers provided thereover upon the substrate12. A first level electroluminescent electrode portion 30 is depositeddirectly upon the insulating substrate 12 during one of the metal vapordeposition stages. The partially fabricated panel is thereafter removedfrom the vacuum system and a laminated photo-polymerizable layer ispressed over the entire panel area. The laminated photo-polymerizablelayer is an insulating material which can be polymerized in place uponexposure to the photo-radiation. A suitable laminated photoresist is"piston," a DuPont trademarked material. This polymerized insulatorlayer 32 is a relatively thick layer typically being about 1 mil inthickness of effectively insulate the thin film circuit elements and thevarious buses from the electroluminescent phosphor layer 34 which coversthe entire panel. It has been the practice to merely contact the firstlevel electroluminescent electrode 30 with the phosphor material. Thus,only that portion of the electroluminescent phosphor layer directlyabove the electrode deposited on the substrate was actually excited toluminescence. A common top electroluminescent electrode 36, which islight transmissive, is disposed atop the top of the phosphor layer 34.This common electrode 36 is connected to the power supply 28. A glassface plate 42 may be provided over top electrode 36. The insulatingpolymer layer 32 covers all of the thin film circuit components exceptthe first level electroluminescent electrodes 30. Apertures are providedin this polymer insulating layer 32 by a further resist technique toexpose the first level electrodes. In these cross-sectional views, therelative dimensions of the layers is greatly exaggerated for ease ofdescription.

In order to further expand the active area of the electroluminescentphosphor a second level electroluminescent electrode 38 is depositedatop the polymer insulator layer 32. This second levelelectroluminescent electrode 38 is generally planar and parallel to thefirst level electroluminescent electrode. A connecting electrode portion40 electrically connecting the first level electroluminescent electrodeand the second level electroluminescent electrode is deposited along theslope of the polymer insulator layer 32 at the edges of the aperturewhich is opened in the polymer insulator layer about the electrode 30.The first level electrode portion 30 is a generally rectangular padwhich occupies a portion of the area of each unit display element. Theapertures which are formed in the polymer insulator layer 32 arerectangular and have a generally inverted cone cross section, i.e., theaperture area is smaller at surface of the first level electrode 30 thanit is at the top surface of the layer 32. A gradual inward sloped edgeis thus formed in layer 32 at each said aperture above each electrode30. The electroluminescent phosphor layer 34 is thereby deposited overthe entire display panel in contact with the first levelelectroluminescent electrode 30, the second level electroluminescentelectrode 38 and the connecting electrode portion 40, so that a greatlyincreased area of phosphor for a given display element is activated.This greatly improves the brightness level of the panel. The individualelectroluminescent electrodes can thus cover a substantial portion ofthe total area of the display panel, with the only area of non-coveragebeing the requisite spacing between adjacent edges of the individualelectrodes. In achieving about seventy percent lit area the spacingbetween electrodes was about 8 mils, this spacing can be reduced toabout 2-3 mils to further increase the electrode area and the lit areaof the panel.

The fabrication of the multi-level electroluminescent electrodestructure in the display panel of the present invention involves carefulattention in deposition of the connecting electrode portion 40 to ensurea continuous, large area electroluminescent electrode made up of thefirst level and second level and connecting electrode portions. Thethickness of the laminated photo-polymerizable insulating layer 32presents a problem in that when the photo-polymerizable material isexposed to photo-radiation with a mask imposed over the firstelectroluminescent electrode area when the apertures are formed over theelectrodes 30 an edge effect is produced at the edge of the mask due todiffraction or scattering of the photo-radiation to produce what afterdeveloping of the unexposed area and forming of the aperture 44 is anoverhang of polymerized material at the upper surface of layer 32 asseen in FIG. 5a. The formed aperture 44 has a smaller area at the uppersurface of layer 32 than at the bottom surface. This overhang ofphoto-polymerized insulating material is undesirable in that it impedesdeposition of an effective connecting electrode portion 40. This isbecause the metal layer which is vacuum deposited as the connectingelectrode portion 40 is done by a line of sight vacuum deposition, andthe overhang of photo-polymerized material prevents deposition of themetal in this overhang area. The edge of the photo-polymerized insulatoradjacent the first level electroluminescent electrode desirably has agradual slope as seen in FIG. 5b where the aperture 46 has the desiredsloped edges with the aperture area at the upper surface of layer 32exceeding the aperture area at the bottom surface to thereby permit lineof sight metal deposition on these edges. The second levelelectroluminescent electrode 38 is vapor deposited at the same time asconnecting electrode portion 40 atop the top planar surface of thephoto-polymer insulator layer 32. The individual electroluminescentelectrodes are preferably formed of aluminum which is vacuum depositedto a thickness of about 1500 Angstroms in forming such electrodes. Thefirst level portion 30 may be thicker than the second level portion 38and the connecting portion 40 because during deposition of the secondlevel and connecting portions the deposited metal also covers thealready deposited first level electrode. This ensures good contactbetween each electrode portion.

A novel technique for ensuring that the slope of the edge of theinsulator layer is as seen in FIG. 5b is illustrated by a novel methodwhich can be best understood by reference to FIG. 4. The photo-radiationtypically ultra-violet used for selectively polymerizing layer 32 isfirst directed through the substrate 12 passed the opaque first levelelectrode 30 itself as well as past the opaque thin film circuitportions. The photo-radiation directed through the substrate passesaround the rectangular first level electrode 30 and photo-polymerizesthe layer 32 with an edge profile about the electrode which approximatesthat seen in FIG. 5b. The bottom portion of layer 32 is fullypolymerized while the upper portions of layer 32 above the edge ofelectrode 30 are unpolymerized so that when the layer 32 is developed toremove the unexposed areas the aperture 46 formed has a smaller area atthe bottom than at the top surface with a gradual sloped edge provided.The sloped edge is thus available for direct line of sight metaldeposition to lay down a continuous connecting electrode portion on thissloped edge.

After the exposure through the substrate 12 to establish the desiredaperture shape, and before developing the layer to form the aperture, asecond photo-exposure is carried out from the conventional direction,i.e., from the top surface of the layer 32. An opaque photomask 48 isaligned over the first level electrode areas. The size or area of theopaque mask aligned above the first level electroluminescent electrodeis greater than the first level electrode area. This is to preventfurther exposure of the upper portion of layer 32 at the edges about theelectrode 30 while at the same time exposing the layer 32 above the restof the panel which were blocked by the opaque buses and the thin filmcircuitry when the photo-radiation was directed through the substrate.The layer 32 is then effectively polymerized at all areas except thearea over the electrode 30.

The plural electrode level panel structure of the present inventionprovides a significant increase in the lit area of the phosphor layerand this increase in area produces an increased brightness panel. Thebrightness of the panel is further improved due to the fact that thesecond level portion of the electrode being atop the relatively thickinsulating layer over the thin film circuitry. The phosphor layerbetween the second level electrode portion and the top electrode is lessthan between the first level electrode portion and the applied voltageacross the thinner phosphor layer produces greater luminescent outputfrom the phosphor.

We claim:
 1. Method of fabricating an electroluminescent display panelcomprised of a thin planar insulating substrate having an X-Y array ofdisplay elements thereon each of which display elements include integralthin film transistor switching and control circuit elements disposed onthe insulating substrate and interconnected by drive signal buses withan electrically insulative polymerized layer disposed over the thin filmcircuit elements and drive signal buses, and each display elementincluding a plural level electroluminescent electrode disposed on theinsulating substrate and extending over the insulative polymerizedlayer, with an electroluminescent phosphor layer disposed over theentire panel area in contact with the electroluminescent electrodes,which method comprises:(a) depositing the interconnected thin filmcircuit elements, signal buses and electroluminescent electrodes insequence as an X-Y array on the planar insulating substrate; (b)laminating the relatively thick insulative photo-polymerizable layerover the entire panel side on which the thin film circuit elements aredisposed; (c) directing photo-radiation through the substrate tophoto-polymerize exposed areas of the photo-polymerizable layer whichare not shielded by the opaque first level electrode and by the opaquethin film circuitry and buses, with the photo-polymerized area of thelayer about the first level electrode at the bottom surface of thephoto-polymerizable layer exceeding the area that is photo-polymerizedat the upper surface of the layer so that a generally inverted conicunpolymerized layer portion extends above each first level electrode;(d) disposing an opaque photomask over the layer aligned with each firstlevel electrode, with the opaque photomask area exceeding the firstlevel electrode area and approximate the area of the unexposed uppersurface above the first level electrode; (e) exposing the layer tophoto-radiation directed from above the photomask to photo-polymerizethe unexposed portion of the layer except for the area covered by thephotomask; (f) removing unpolymerized portion of the layer; (g) vacuumdepositing conductive metal electrodes over the insulativephoto-polymerized layer to form the second level electrode portions andthe connecting electrode portion which extend down to and contact thefirst level electrode portion; and (h) depositing an electroluminescentphosphor layer on the conductive metal electrodes and connectingelectrode portions and onto the insulative photo-polymerized layerbetween the spaced electrode portions.